I'm a DJ. I want a DJ interface for Spotify, where I can play two tracks at once over the top of each other.
It doesn't have to be anything complex -- just two tracks and a crossfader would do the job. But if you fancy adding things like waveforms, pitch-shifting, and other functionality, that would be lovely too.
In 2014 our friends at Algoriddim announced a major update to djay for iPhone and iPad. This new version lets all our Premium subscribers mix millions of tracks using their iPhone and iPad, as well as hook up their professional DJ hardware to a massive library of music. You can read more about djay and Spotify right here.
In case you're interested there are separate Ideas for other platforms here: